1. Field of the Invention
The present invention relates to an antenna, particularly a compact antenna suitable for inclusion in various devices having capabilities for processing radio signals, including various communication devices that can transmit and receive radio signals.
2. Description of the Related Art
In recent years, there have been increasing uses for antennas that can be used in frequency bands in a range of several hundreds of MHz to several tens of GHz due to increasing demand for various devices having capabilities for transmitting and receiving radio signals, including various communication devices for processing radio signals. Obvious uses for such antennas include mobile communications, next generation traffic management systems, non-contacting type cards for automatic toll collection systems, but in addition, because of the trend toward the use of wireless data handling systems that enable to handle data, without using cumbersome lengthy cables, such as cordless operation of household appliances through the Internet, Intranet radio LAN, Bluetooth and the like, it is anticipated that the use of such antennas will also be widespread in similar fields. Furthermore, such antennas are used in various systems for wireless data handling from various terminals, and the demand is also increasing for applications in telemetering for monitoring information on water pipes, natural gas pipelines and other safety management systems and POS (point-of-sale) terminals in financial systems. Other applications are beginning to emerge over a wide field of commerce including household appliances such as TV that can be made portable by satellite broadcasting as well as vending machines.
To date, such antennas described above used in various devices having capabilities for receiving and transmitting radio signals are mainly monopole antennas attached to the casing of a device. Also known are helical antennas that protrude slightly to the exterior of the casing.
However, in the case of monopole antennas, it is necessary to extend the structure for each use of the device to make the operation cumbersome, and, there is a further problem that the extended portion is susceptible to breaking. Also, in the case of the helical antennas, because a hollow coil that serves as the antenna main body is embedded in a covering material such as polymer resin for protection, the size of device tends to increase if it is mounted on the outside the casing and it is difficult to avoid the problem that the aesthetics suffers. Nevertheless, reducing the size of the antenna leads only to lowering of signal gain, which inevitably leads to increasing the circuit size for processing radio signals to result in significantly higher power consumption and a need for increasing the size of the battery, and ultimately leading back to the problem that the overall size of the device cannot be reduced.
However, when attempts are made to realize a compact antenna comprised by a resonant circuit having an inductance section and a capacitance section, it is difficult to obtain sufficient inductance values, and even if a coil-shaped antenna is used, there is a problem that the area of the opening cannot be made large. For example, although a coil design is known that utilizes conductor patterns formed on front and back surfaces of a substrate plate, which are connected electrically via a through-hole, in this case, the coil opening area is limited by the dimensions of the thickness and width of the substrate plate. Naturally, by increasing the thickness and width of the substrate plate, the size of the opening area can be made larger, but this approach does not enable to reduce the antenna size. Also, increasing the number of winding of the coil naturally increases inductance values, but for high frequency applications, the conductor patterns must be separated to some extent, such that increasing the number of windings leads to lengthening the antenna.
The present invention is provided in view of the background information described above, and an object is to provide a compact antenna that enables to raise the inductance values of the resonant section and to obtain high gain.
A first embodiment of the present invention relates to an antenna comprising a resonance section having an inductance section and a capacitance section connected electrically in parallel; wherein the inductance section has a coil section comprised by a conductor formed in a spiral shape circling a coil axis or an angular shape that can be approximated by a spiral circling the coil axis, and at least one opening section of opening sections formed at both ends of the coil section is contained in a plane oriented at an angle to the coil axis.
By having such a structure, the area of the opening section is increased and at the same time, the magnetic flux penetrating through the opening section is also increased, such that inductance values of the coil section is increased.
The conductor is formed by linking the portion that circles the coil axis in plurality in the direction of the coil axis. If cylindrical coordinates are used to designate the coil axis as z-axis, and describe the position of each section of the conductor, a typical spiral exhibits monotonic changes in the z-coordinate as the angular coordinate xcex8 is varied. Then, consider a spiral conductor that circles the coil axis over an angular displacement of xcex8=360 degrees, and one plane intersecting the z-axis at right angles at the starting point and another plane intersecting the z-axis at the ending point of such a spiral, then this spiral does not intersect the planes except at the beginning point and at the ending point of the conductor spiral. If one supposes such a plane for each complete revolution (or turning portion) of the conductor spiral, then the conductor is divided by a series of such planes at right angles to the coil axis. When this argument is extended to a general spiral-like conductor or a conductor that can be approximated by a spiral, a group of such planes can be visualized to divide the conductor but the turning portions (loops) of the conductor do not intersect the planes except at the beginning points and the ending points of each loop. Then, the portion that circles the coil axis of the conductor can be associated with an adjacent imaginary plane that separates the portion, so that an expression xe2x80x9cthe portion that circles the coil axis is substantially contained within the imaginary plane that divides the conductorxe2x80x9d is used. (herein below imaginary planes that divide the conductor are referred to simply as planes). The opening sections formed at both ends of the coil section is comprised by the portion that circles the coil axis, and the opening section is substantially contained within the plane that substantially contains the portion circling the coil axis.
It can be seen that, when the opening section is contained within the plane oriented at an angle to the coil axis, the orientation of the magnetic field produced by the current flowing in this portion of the coil is generated substantially at right angles to the coil axis. The magnetic flux that penetrates this inclined plane is higher than a case of similar magnetic flux that penetrate a plane at right angles to the coil axis. It thus follows that the inductance value of the coil section is increased.
In this case, it is preferable that respective portions of the conductor that circle the coil axes are provided parallel to the opening section contained in a plane oriented at an angle to the coil axis. By adopting this structure, the magnetic flux penetrating the plane that includes the portion circling the coil axis of the conductor is also increased, and the inductance values are further increased.
Also, it is preferable that the antenna has a plurality of resonance sections, and the resonance sections are connected electrically in series. By adopting this structure, the gain of the antenna is increased.
Additionally, it is preferable that, in at least two adjacent resonance sections, coil axes of the respective coil sections are aligned on a straight line; and the planes that substantially contain the opening sections of adjacent coil sections are oriented at right angles to each other. By adopting this structure, the two coil sections are aligned on the same straight line so that the mounting area of the antenna is reduced, and because the direction of the magnetic field for a maximum magnetic flux through the one coil is perpendicular to the direction of the magnetic field for a maximum magnetic flux through the other coil, antenna gain is effective for both the vertically and horizontally polarized signal waves.
To summarize the features of the present invention, the following beneficial effects are noted.
As explained above, according to the present invention, the antenna has a resonance section having an inductance section and a capacitance section connected electrically in parallel, and the inductance section has a coil section, and at least one of the openings provided at both ends of the coil section is contained in a plane oriented at an angle to the coil axis so that the inductance value of the coil section is increased, and the antenna gain can be increased without unduly increasing the total length of the antenna.
Also, according to the present invention, the portion that circles the coil axis of the conductor is provided parallel to the opening section that is substantially contained in a plane oriented at an angle to the coil axis so that the value of inductance of the coil section is further increased, and the antenna gain can be increased without unduly increasing the total length of the antenna.
Also, according to the present invention, because the antenna is constructed of a plurality of resonance sections connected electrically in series, the antenna gain can be increased.
Further, according to the present invention, because the antenna is constructed in such a way that a plurality of resonance sections are connected electrically in series by aligning the coil axes of the adjacent coil sections approximately on a straight line, and that the planes containing the opening sections of the adjacent coil sections are oriented at about the right angles to each other, the antenna gain for vertically polarized waves and horizontally polarized waves can be obtained using a small mounting area.